Screening apparatus are known in which a screen gap is formed between a pair of relatively movable parts. The operating efficiency of such apparatus can be deleteriously affected if material suspensions clog the screen gap, thereby inhibiting the screen gap from effectively performing its screening function.
In the past, efforts have been made to prevent the clogging of the screen gaps of these apparatus. For instance, Haug U.S. Pat. No. 1,185,794 describes and illustrates a screening machine designed to automatically clean screen openings formed between relatively movable parts. Inasmuch as the screening machine of the Haug patent relies solely on the relative movement of the parts to cause stock, which cannot pass through the screen openings, to bypass the screen openings, there is nothing which positively inhibits clogging of the screen openings by rejecting or defibrating the stock. Thus, the screening machine of the Haug patent suffers from the problem that some of the stock might not bypass the screen openings, which could therefore become clogged by the unbypassed stock.